1. Field of the Invention
The present invention relates to a semiconductor laser device having a current confinement structure and an index-guided structure, and a process for producing a u semiconductor light emitting device having a current confinement structure and an index-guided structure.
2. Description of the Related Art
(1) In many conventional current semiconductor laser devices which emit light in the 0.9 to 1.1 xcexcm band, a current confinement structure and index-guided structure are provided in crystal layers which constitute the semiconductor laser devices so that the semiconductor laser device oscillates in a fundamental transverse mode. For example, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp.102 discloses a semiconductor laser device which emits light in the 0.98 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al0.48Ga0.52As lower cladding layer, an undoped Al0.2Ga0.8As optical waveguide layer, an Al0.2Ga0.8As/In0.2Ga0.8As double quantum well active layer, an undoped Al0.2Ga0.8As optical waveguide layer, a p-type AlGaAs first upper cladding layer, a p-type Al0.67Ga0.33As etching stop layer, a p-type Al0.48Ga0.52As second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above the p-type Al0.67Ga0.33As etching stop layer by normal photolithography and selective etching, and an n-type Al0.7Ga0.3As and n-type GaAs materials are embedded in both sides of the ridge structure by selective MOCVD using Cl gas. Then, the insulation film is removed, and thereafter a p-type GaAs layer is formed. Thus, a current confinement structure and an index-guided structure are built in the semiconductor laser device.
However, the above semiconductor laser device has a drawback that it is very difficult to form the AlGaAs second upper cladding layer on the AlGaAs first upper cladding layer, since the AlGaAs first upper cladding layer contains a high Al content and is prone to oxidation, and selective growth of the AlGaAs second upper cladding layer is difficult.
(2) In addition, IEEE Journal of Quantum Electronics, vol. 29, No. 6, 1993, pp.1936 discloses a semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 to 1.02 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type Al0.4Ga0.6As lower cladding layer, an undoped Al0.2Ga0.8As optical waveguide layer, a GaAs/InGaAs double quantum well active layer, an undoped Al0.2Ga0.8As optical waveguide layer, a p-type Al0.4Ga0.6As upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above a mid-thickness of the p-type Al0.4Ga0.6As upper cladding layer by normal photolithography and selective etching, and an n-type In0.5Ga0.5P material and an n-type GaAs material are embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and then electrodes are formed. Thus, a current confinement structure and an index-guided structure are realized in the layered construction.
However, the above semiconductor laser device also has a drawback that it is very difficult to form the InGaP layer on the AlGaAs upper cladding layer, since the AlGaAs upper cladding layer contains a high Al content and is prone to oxidation, and it is difficult to grow an InGaP layer having different V-group component, on such an upper cladding layer.
(3) Further, IEEE Journal of Selected Topics in Quantum Electronics, vol. 1, No. 2, 1995, pp.189 discloses an all-layer-Aluminum-free semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.98 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type InGaP cladding layer, an undoped InGaAsP optical waveguide layer, an InGaAsP tensile strain barrier layer, an InGaAs double quantum well active layer, an InGaAsP tensile strain barrier layer, an undoped InGaAsP optical waveguide layer, a p-type InGaP first upper cladding layer, a p-type GaAs optical waveguide layer, a p-type ten InGaP second upper cladding layer, a p-type GaAs cap layer, and an insulation film are formed in this order. Next, a narrow-stripe ridge structure is formed above or the p-type InGaP first upper cladding layer by normal photolithography and selective etching, and an n-type In0.5Ga0.5P material is embedded in both sides of the ridge structure by selective MOCVD. Finally, the insulation film is removed, and a p-type GaAs contact layer is formed. Thus, a current confinement structure and an index-guided structure are realized.
The reliability of the above semiconductor laser device is improved since the strain in the active layer can be compensated for. However, the above semiconductor laser device also has a drawback that the kink level is low (about 150 mW) due to poor controllability of the ridge width.
(4) Furthermore, IEEE Journal of Quantum Electronics, vol. 29, No. 6, 1993, pp.1889 discloses an internal striped structure semiconductor laser device which oscillates in a fundamental transverse mode, and emits light in the 0.8 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type AlGaAs lower cladding layer, an AlGaAs/GaAs triple quantum well active layer, a p-type AlGaAs first upper cladding layer, an n-type AlGaAs current confinement layer, and an n-type AlGaAs protection layer are formed in this order. Next, a narrow-stripe groove is formed, by normal photolithography and selective etching, to such a depth that the groove penetrates the n-type AlGaAs current confinement layer. Next, over the above structure, a p-type AlGaAs second upper cladding layer and a p-type GaAs contact layer are formed.
In the above semiconductor laser device, the stripe width can be controlled accurately, and high-output-power oscillation in a fundamental transverse mode can be realized by the difference in the refractive index between the n-type AlGaAs current confinement layer and the p-type AlGaAs second upper cladding layer. However, the above semiconductor laser device also has a drawback that it is difficult to form an AlGaAs layer on another AlGaAs layer since the AlGaAs layers are prone to oxidation.
As described above, the conventional current semiconductor laser devices which include a current confinement structure and an index-guided structure, oscillate in a fundamental transverse mode, and emit light in the 0.9 to 1.1 xcexcm band with high output power, are unreliable, or uneasy to produce, or have poor characteristics.
(5) Alternatively, in many conventional current semiconductor laser devices which emit light in the 0.9 to 1.1 xcexcm band, a current confinement structure is provided in crystal layers which constitute the semiconductor laser devices so as to oscillate in a fundamental transverse mode. For example, Proceedings of SPIE, vol. 3628, 1999, pp.38-45 discloses an internal striped structure semiconductor laser device which emits light in the 0.98 xcexcm band. This semiconductor laser device is formed as follows.
On an n-type GaAs substrate, an n-type AlxGa1-xAs lower cladding layer, an n-type GaAs optical waveguide layer, an InGaAs quantum well active layer, a p-type GaAs first upper optical waveguide layer, and an n-type AlyGa1-yAs current confinement layer are formed in this order. Next, a narrow-stripe groove is formed, by normal photolithography and selective etching, to such a depth that the groove penetrates the n-type AlGaAs current confinement layer. Next, over the above structure, a GaAs second optical waveguide layer, a p-type AlGaAs upper cladding layer, and a p-type GaAs contact layer are formed.
In the above semiconductor laser device, the stripe width can be controlled accurately, and high-output-power oscillation in a fundamental transverse mode can be realized by the difference in the refractive index between the n-type AlGaAs current confinement layer and the p-type GaAs second upper cladding layer. However, the above semiconductor laser device also has a drawback that it is difficult to form an AlGaAs layer on another AlGaAs layer since the AlGaAs layers are prone to oxidation. In addition, since the optical waveguide layer is made of GaAs, there is large current leakage, and the threshold current becomes high, although AlGaAs leak-current protection layers are provided on both sides of the active layer.
As in the above example, the conventional current semiconductor laser devices which contain a current confinement structure, and oscillate in a fundamental transverse mode are also unreliable, or uneasy to produce, or have poor characteristics, and the above conventional current semiconductor laser devices cannot oscillate in a fundamental transverse mode when output power is high.
An object of the present invention is to provide a reliable semiconductor laser device which can oscillate in a fundamental transverse mode even when output power is high.
Another object of the present invention is to provide a process for producing a reliable semiconductor laser device which can oscillate in a fundamental transverse mode even when output power is high.
(1) According to the first aspect of the present invention, there is provided a semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer formed on the lower cladding layer; a compressive strain quantum well active layer made of Inx3Ga1-x3As1-y3Py3, and formed on the lower optical waveguide layer, where 0 less than x3xe2x89xa60.4, 0xe2x89xa6y3xe2x89xa60.1, and the absolute value of a first product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; an upper optical waveguide layer formed on the Inx3Ga1-x3As1-y3Py3 compressive strain quantum well active layer; a first upper cladding layer made of In0.49Ga0.51P of a second conductive type, and formed on the upper optical waveguide layer; an etching stop layer made of Inx1Ga1-x1As1-y1Py1, and formed on the first upper cladding layer other than a stripe area of the first upper cladding layer so as to form a first portion of a stripe groove realizing a current injection window, where 0xe2x89xa6x1xe2x89xa60.3, 0xe2x89xa6y1xe2x89xa60.6, and the absolute value of a second product of the strain and the thickness of the etching stop layer is equal to or smaller than 0.25 nm; a current confinement layer made of In0.49Ga0.51P of the first conductive type, and formed on the etching stop layer so as to form a second portion of the stripe groove; a second upper cladding layer made of Inx4Ga1-y4As1-y4Py4 of the second conductive type, and formed on the current confinement layer and the stripe area of the first upper cladding layer, where x4=(0.49xc2x10.01)y4 and 0.4xe2x89xa6x4xe2x89xa60.46; and a contact layer of the second conductive type, formed on the second upper cladding layer. In the semiconductor laser device, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate.
The first conductive type is different in carrier polarity from the second conductive type. That is, when the first conductive type is n type, and the second conductive type is p type.
The strain of the compressive strain quantum well active layer is defined as (caxe2x88x92cs)/cs, where cs and ca are the lattice constants of the GaAs substrate and the compressive strain quantum well active layer, respectively.
The strain of the etching stop layer is defined as (cexe2x88x92cs)/cs, where cs and ce are the lattice constants of the GaAs substrate and the etching stop layer, respectively.
When a layer grown over the substrate has a lattice constant c, and an absolute value of the amount (cxe2x88x92cs)/cs is equal to or smaller than 0.003, the layer is lattice-matched with the substrate. That is, in the semiconductor laser devices according to the first and second aspects of the present invention, the absolute value of the amount (cxe2x88x92cs)/cs is equal to or smaller than 0.003 for each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, and the second upper cladding layer.
Preferably, the semiconductor laser device according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iv).
(i) The semiconductor laser device may further include first and second tensile strain barrier layers both made of Inx5Ga1-x5As1-y5Py5, and respectively formed above and below the compressive strain quantum well active layer, where 0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6, and the absolute value of a sum of the first product and a third product of the strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm. The strain of the first and second tensile strain barrier layers is defined as (cbxe2x88x92cs)/cs, where cb is the lattice constant of the first and second tensile strain barrier layers.
(ii) The etching stop layer may be one of the first and second conductive types.
(iii) The stripe groove may have a width equal to or greater than 1 xcexcm.
(iv) The compressive strain quantum well active layer may include a plurality of quantum wells.
(2) According to the second aspect of the present invention, there is provided a semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer formed on the lower cladding layer; a compressive strain quantum well active layer made of Inx3Ga1-x3As1-y3Py3, and formed on the lower optical waveguide layer, where 0 less than x3xe2x89xa60.4, 0xe2x89xa6y3xe2x89xa60.1, and the absolute value of a first product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; an upper optical waveguide layer formed on the Inx3Ga1-x3As1-y3Py3 compressive strain quantum well active layer; a first upper cladding layer made of In0.49Ga0.85P of a second conductive type, and formed on the upper optical waveguide layer; an etching stop layer made of Inx1Ga1-x1As1-y1Py1 of the second conductive type, and formed on the first upper cladding layer, where 0xe2x89xa6x1xe2x89xa60.3, 0xe2x89xa6y1xe2x89xa60.6, and the absolute value of a second product of the strain and the thickness of the etching stop layer is equal to or smaller than 0.25 nm; a current confinement layer made of In0.49Ga0.51P of the first conductive type, and formed on the etching stop layer other than a stripe area of the etching stop layer so as to form a stripe groove realizing a current injection window; a second upper cladding layer made of Inx4Ga1-x4A1-y4Py4 of the second conductive type, and formed on the current confinement layer and the stripe area of the etching stop layer, where x4=(0.49xc2x10.01)y4 and 0.4xe2x89xa6x4xe2x89xa60.46; and a contact layer of the second conductive type, formed on the second upper cladding layer. In the semiconductor laser device, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate.
Preferably, the semiconductor laser device according to the second aspect of the present invention may also have one or any possible combination of the following additional features (v) to (vii).
(v) The semiconductor laser device may further include first and second tensile strain barrier layers both made of Inx5Ga1-x5As1-y5Py5 and respectively formed above and below the compressive strain quantum well active layer, where 0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6, and the absolute value of a sum of the first product and a third product of the strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.
(vi) The stripe groove may have a width equal to or greater than 1 xcexcm.
(vii) The compressive strain quantum well active layer may include a plurality of quantum wells.
(3) According to the third aspect of the present invention, there is provided a process for producing a semiconductor laser device, including the steps of: (a) forming a lower cladding layer of a first conductive type on a GaAs substrate of the first conductive type; (b) forming a lower optical waveguide layer on the lower cladding layer; (c) forming a compressive strain quantum well active layer made of Inx3Ga1-x3As1-y3Py3, on the lower optical waveguide layer, where 0 less than x3xe2x89xa60.4, 0xe2x89xa6y3xe2x89xa60.1, and the absolute value of a first product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; (d) forming an upper optical waveguide layer on the Inx3Ga1-x3As1-y3Py3 compressive strain quantum well active layer; (e) forming a first upper cladding layer made of In0.49Ga0.51P of a second conductive type, on the upper optical waveguide layer; (f) forming an etching stop layer made of Inx1Ga1-x1As1-y1Py1, on the first upper cladding layer, where 0xe2x89xa6x1xe2x89xa60.3 and 0xe2x89xa6y1xe2x89xa60.6, and the absolute value of a second product of the strain and the thickness of the etching stop layer is equal to or smaller than 0.25 nm; (g) forming a current confinement layer made of In0.49Ga0.51P of the first conductive type, on the etching stop layer; (h) removing a stripe area of the current confinement layer so as to form a stripe groove for realizing a current injection window; (i) forming a second upper cladding layer made of Inx4Ga1-x4As1-y4Py4 of the second conductive type so that the stripe groove is covered with the second upper cladding layer, where x4=(0.49xc2x10.01)y4 and 0.4xe2x89xa6x4xe2x89xa60.46; and (j) forming a contact layer of the second conductive type, on the second upper cladding layer. In the process, each of the lower cladding layer, the lower optical waveguide layer, the upper optical waveguide layer, the first upper cladding layer, the current confinement layer, the second upper cladding layer, and the contact layer has such a composition as to lattice-match with the GaAs substrate.
That is, the semiconductor laser device according to the second aspect of the present invention can be produced by the process according to the third aspect of the present invention.
Preferably, the process according to the third aspect of the present invention may also have one or any possible combination of the following additional features (viii) to (xi).
(viii) The process may further include the steps of: (b1) after the step (b), forming a first tensile strain barrier layer made of Inx5Ga1-x5As1-y5Py5, on the lower optical waveguide layer, where 0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6; and (c1) after the step (c), forming a second tensile strain barrier layer made of Inx5Ga1-x5As1-y5Py5, on the compressive strain quantum well active layer; where the absolute value of a sum of the first product and a third product of the strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.
(ix) The process may further include, after the step (g), the steps of: (g1) forming a cap layer made of GaAs; and (g2) removing a stripe area of the cap layer; and the process may further include, after the step (h), the step of (h1) removing a remaining area of the cap layer and a stripe area of the etching stop layer so as to form an additional portion of the stripe groove. That is, the semiconductor laser device according to the first aspect of the present invention can be produced by the process according to the third aspect of the present invention when the process includes the above steps (g1), (g2), and (h1). When a GaAs cap layer is used as in the above steps (g1), (g2), and (h1), it is possible to prevent formation of a natural oxidation film on the InGaP current confinement layer, and metamorphic change in the InGaP current confinement layer, which may occur when a resist layer is formed directly on the InGaP current confinement layer. In addition, since the GaAs cap layer is removed before the second upper cladding layer is formed, it is possible to remove a residue left on a boundary surface on which the second upper cladding layer is formed, and prevent the occurrence of crystal defects.
(x) The cap layer may be one of the first and second conductive types.
(xi) The etching stop layer may be one of the first and second conductive types.
(4) The first and second aspects of the present invention have the following advantages.
(a) In the semiconductor laser devices according to the first and second aspects of the present invention, the current confinement layer is made of In0.49Ga0.51P, and the second upper cladding layer is made of Inx4Ga1-x4As1-y4Py4. Therefore, the difference in the refractive indexes between the current confinement layer and the second upper cladding layer realizes a difference of about 1.5xc3x9710xe2x88x923 to 7xc3x9710xe2x88x923 in the equivalent refractive index of the active layer between the portion under the current confinement layer and the portion a under the stripe groove, with high accuracy, and it is possible to cut off oscillation in higher modes. Thus, oscillation in the fundamental transverse mode can be maintained even when the output power becomes high.
(b) When aluminum exists near a boundary surface on which the second upper cladding layer is formed, the boundary surface is prone to oxidation, and it is difficult to realize desired characteristics in the semiconductor laser device. However, the In0.49Ga0.51P first upper cladding layer, the Inx1Ga1-x1As1-y1Py1 etching stop layer, or the In0.49Ga0.51P current confinement layer, which can be exposed at a boundary surface on which the second upper cladding layer is formed, do not include aluminum. Therefore, it is easy to form the second upper cladding layer. In addition, since no crystal defect due to oxidation of aluminum occurs, the characteristics of the semiconductor laser device do not deteriorate, and reliability is improved.
(c) Since the current confinement layer is arranged within the semiconductor laser device, it is possible to increase the contact area between the electrode and the contact layer. Therefore, the contact resistance can be reduced.
(d) Since the etching stop layer is made of InGaAsP, controllability of the stripe width by wet etching is enhanced.
(e) Although the InGaAsP etching stop layer does not have a stripe opening for the current injection window in the semiconductor laser device according to the second aspect of the present invention, the semiconductor laser device according to the second aspect of the present invention has the same advantages as the semiconductor laser device according to the first aspect of the present invention.
(f) When the tensile strain barrier layers are provided as described in the paragraphs (1)(i) and (2)(v), various characteristics are improved (e.g., the threshold current is lowered), and reliability is increased.
(g) When the stripe width is equal to or greater than 1 xcexcm, as described in paragraphs (1)(iii) and (2)(vi), the semiconductor laser device can oscillate in multiple modes with high output power and low noise.
(5) According to the fourth aspect of the present invention, there is provided a semiconductor laser device including: a GaAs substrate of a first conductive type; a lower cladding layer of the first conductive type, formed on the GaAs substrate; a lower optical waveguide layer, and formed on the lower cladding layer; a compressive strain quantum well active layer made of Inx3Ga1-x3As1-y3Py3, and formed on the lower optical waveguide layer, where 0 less than x3xe2x89xa60.4 and 0xe2x89xa6y3xe2x89xa60.1; a first upper optical waveguide layer made of Inx2Ga1-x2As1-y2Py3, and formed on the Inx3Ga1-x3As1-y3Py3 compressive strain quantum well active layer, where x2=(0.49xc2x10.01)y2 and 0xe2x89xa6x2xe2x89xa60.3; a first etching stop layer made of Inx6Ga1-x6P of a second conductive type, and formed on the first upper optical waveguide layer, where 0.2xe2x89xa6x6xe2x89xa60.8; a second etching stop layer made of Inx4Ga1-x1As1-y1Py1, and formed on the first etching stop layer other than a stripe area of the first etching stop layer so as to form a first portion of a stripe groove realizing a current injection window, where 0xe2x89xa6x1xe2x89xa60.3 and 0xe2x89xa6y1xe2x89xa60.6; a current confinement layer made of In0.49Ga0.51P of the first conductive type, and formed on the second etching stop layer so as to form a second portion of the stripe groove; a second upper optical waveguide layer made of Inx2Ga1-x2As1-y2Py2 of the second conductive type so as to cover the stripe groove, where x2=(0.49xc2x10.01)y2 and 0xe2x89xa6x2xe2x89xa60.3; an upper cladding layer made of Inx4Ga1-x4As1-y4Py4 of the second conductive type, and formed over the second upper optical waveguide layer, where x4=(0.49xc2x10.01)y4 and 0.9xe2x89xa6y4xe2x89xa61; and a contact layer of the second conductive type, formed on the upper cladding layer. In the semiconductor laser device, the absolute value of a first product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm, the absolute value of a sum of a second product and a third product is equal to or smaller than 0.25 nm, where the second product is a product of the strain and the thickness of the first etching stop layer, and the third product is a product of the strain and the thickness of the second etching stop layer, and each of the lower cladding layer, the lower optical waveguide layer, the first and second upper optical waveguide layers, the current confinement layer, the upper cladding layer, and the contact layer is formed to have such composition as to lattice-match with the GaAs substrate.
Preferably, the semiconductor laser device according to the fourth aspect of the present invention may also have one or any possible combination of the following additional features (xii) to (xv).
(xii) The semiconductor laser device may further include first and second tensile strain barrier layers both made of Inx5Ga1-x5As1-y5Py5, and respectively formed above and below the compressive strain quantum well active layer, where 0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6, and the absolute value of a sum of the first product and a fourth product of the strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.
(xiii) The second etching stop layer may be one of the first and second conductive types.
(xiv) The stripe groove may have a width equal to or greater than 1 xcexcm.
(xv) The compressive strain quantum well active layer may include a plurality of quantum wells.
(6) According to the fifth aspect of the present invention, there is provided a process for producing a semiconductor laser device, including the steps of: (a) forming a lower cladding layer of a first conductive type on a GaAs substrate of the first conductive type; (b) forming a lower optical waveguide layer on the lower cladding layer; (c) forming a compressive strain quantum well active layer made of Inx3Ga1-x3As1-y3Py3, on the lower optical waveguide layer, where 0 less than x3xe2x89xa60.4, 0xe2x89xa6y3xe2x89xa60.1, and the absolute value of a first product of the strain and the thickness of the compressive strain quantum well active layer is equal to or smaller than 0.25 nm; (d) forming a first upper optical waveguide layer made of Inx2Ga1-x2As1-y2Py2, on the Inx3Ga1-x3AS1-y3Py3 compressive strain quantum well active layer, where x2=(0.49xc2x10.01)y2 and 0xe2x89xa6x2xe2x89xa60.3; (e) forming a first etching stop layer made of Inx6Ga1-x6P of a second conductive type, on the first upper optical waveguide layer, where 0.2xe2x89xa6x6xe2x89xa60.8; (f) forming a second etching stop layer made of Inx1Ga1-x1As1-y1Py1, on the first upper cladding layer, where 0xe2x89xa6x1xe2x89xa60.3 and 0xe2x89xa6y1xe2x89xa60.6; (g) forming a current confinement layer made of In0.49Ga0.51P of the first conductive type, on the second etching stop layer; (h) removing a stripe area of the current confinement layer and a stripe area of the second etching stop layer so as to form a stripe groove realizing a current injection window; (i) forming a second upper optical waveguide layer made of Inx2Ga1-x2As1-y2Py2 of the second conductive type, so as to cover the stripe groove, where x2=(0.49xc2x10.01)y2 and 0xe2x89xa6x2xe2x89xa60.3; (j) forming an upper cladding layer made of Inx4Ga1-x4As1-y4Py4 of the second conductive type, over the second upper optical waveguide layer, where x4=(0.49xc2x10.01)y4 and 0.9xe2x89xa6y4xe2x89xa61; (k) forming a contact layer of the second conductive type, on the second upper cladding layer. In the process, the absolute value of a sum of a second product and a third product is equal to or smaller than 0.25 nm, where the second product is a product of the strain and the thickness of the first etching stop layer, and the third product is a product of the strain and the thickness of the second etching stop layer, and each of the lower cladding layer, the lower optical waveguide layer, the first and second upper optical waveguide layers, the current confinement layer, the upper cladding layer, and the contact layer is formed to have such a composition as to lattice-match with the GaAs substrate.
That is, the semiconductor laser device according to the fourth aspect of the present invention can be produced by the process according to the fifth aspect of the present invention.
Preferably, the process according to the fifth aspect of the present invention may also have one or any possible combination of the following additional features (xvi) to (xix).
(xvi) The process may further include the steps of: (b1) after the step (b), forming a first tensile strain barrier layer made of Inx5Ga1-x5As1-y5Py5, on the lower optical waveguide layer, where 0xe2x89xa6x5xe2x89xa60.3 and 0 less than y5xe2x89xa60.6; and (c1) after the step (c), forming a second tensile strain barrier layer made of Inx5Ga1-x5As1-x5Py5, on the compressive strain quantum well active layer, where the absolute value of a sum of the first product and a fourth product of the strain of the first and second tensile strain barrier layers and a total thickness of the first and second tensile strain barrier layers is equal to or smaller than 0.25 nm.
(xvii) The process may further include, after the step (g), the step of (g1) forming a cap layer made of GaAs, and the step (h) comprising the substeps of: (h1) removing a stripe area of the cap layer and the stripe area of the current confinement layer, and (h2) removing a remaining area of the cap layer and the stripe area of the second etching stop layer. When a GaAs cap layer is used as in the above steps (g1) and (h1), it is possible to prevent formation of a natural oxidation film on the InGaP current confinement layer, and metamorphic change in the InGaP current confinement layer, which occurs when a resist layer is formed directly on the InGaP current confinement layer. In addition, since the GaAs cap layer is removed before the second upper optical waveguide layer is formed, it is possible to remove a residue left on the boundary surface on which the second upper optical waveguide layer is formed, and prevent the occurrence of crystal defects.
(xviii) The second etching stop layer may be one of the first and second conductive types.
(xix) The cap layer may be one of the first and second conductive types.
(7) The fifth aspect of the present invention has the following advantages.
(a) In the semiconductor laser device according to the third aspect of the present invention, the current confinement layer is made of In0.49Ga0.51P, and the second upper optical waveguide layer is made of Inx4Ga1-x4As1-y4Py4. Therefore, the difference in the refractive index between the current confinement layer and the second upper optical waveguide layer realizes a difference of 1.5xc3x9710xe2x88x923 to 7xc3x9710xe2x88x923 in the equivalent refractive index of the active layer between the portion under the current confinement layer and the portion under the stripe groove, with high accuracy, and it is possible to cut off oscillation in higher modes. Thus, oscillation in the fundamental transverse mode can be maintained even when the output power becomes high.
(b) Deterioration of light exit end surfaces of semiconductor laser devices which emit laser light with high output power can be effectively prevented by increasing the thickness of the optical waveguide layer, since the peak optical density is reduced when the thickness of the optical waveguide layer is increased. However, in the conventional semiconductor laser devices having a current confinement structure and an index-guided structure, it is impossible to increase the thickness of a portion between the current confinement layer and the active layer since a fundamental transverse mode must be achieved. In particular, the thickness of the optical waveguide layer is limited since the optical waveguide layer is located between the current confinement layer and the active layer on the other hand, in the semiconductor laser device according to the fifth aspect of the present invention, the thickness of the optical waveguide layer is substantially increased by arranging the Inx1Ga1-x1As1-y1Py1 second upper optical waveguide layer above the current confinement layer, where the second upper optical waveguide layer has the same composition as the first upper optical waveguide layer. Therefore, it is possible to reduce the peak optical density and the deterioration of a light exit end surface which is caused by high optical density. Thus, reliability is increased.
(c) Compared with the conventional semiconductor laser device, the difference in the band gap between the active layer and the Inx1Ga1-x1As1-y1Py1 second upper optical waveguide layer can be increased by provision of the Inx1Ga1-x1As1-y1Py1 second upper optical waveguide layer. Therefore, it is possible to prevent leakage currents, and efficiently confine the carriers. Thus, the threshold current can be lowered.
(d) Since the current confinement layer is arranged within the semiconductor laser device, it is possible to increase the contact area between the electrode and the contact layer. Therefore, the contact resistance can be reduced.
(e) The first etching stop layer is made of Inx6Ga1-x6Py1, and the second etching stop layer made of Inx1Ga1-x1As1-y1Py1 is formed on the first etching stop layer. Therefore, when a sulfuric acid etchant is used, only the Inx1Ga1-x6As1-y1Py1 second etching stop layer is etched, and the Inx6Ga1-x6P first etching stop layer is not etched. That is, it is possible to stop etching accurately on the surface of the first etching stop layer, and thus the stripe width can be accurately controlled by wet etching.
(f) Since the In0.49Ga0.51P first upper cladding layer, the Inx1Ga1-x1As1-y1Py1 etching stop layer, or the In0.49Ga0.51P current confinement layer, which can be exposed at a boundary surface on which the second upper optical waveguide layer is formed, does not include aluminum, it is easy to form the second upper optical waveguide layer. In addition, since no crystal defect due to oxidation of aluminum occurs, the characteristics of the semiconductor laser device do not deteriorate, and reliability is improved.
(g) when the tensile strain barrier layers are provided as described in paragraphs (5) (xii), various characteristics are improved (e.g., the threshold current is lowered), and reliability is increased.
(h) When the stripe width is equal to or greater than 1 xcexcm, as described in paragraphs (5) (xiv), the semiconductor laser device can oscillate in multiple modes with high output power and low noise.